Here is what I use most often. First, try to learn the MU as best as you can. If you know what your opponent wants, then you can react to when they are going to do it, rather that what they are doing. Because of this you will know the spacing of where you will be. Walk a little more to slow down the pace of the game. The more patient you are, and the more your opponent gets impatient, and they are going to run in very predictably. Use this to punish them for not thinking. I mostly go for sour spots in neutral because they lead into strings that will combo into a tipper, if you lab it out. Try to get really good at tech chasing, and DB. The most broken thing that Marth has is Tech chasing and Dancing Bade. Marth can tech chase at 0 through 160, depending on weight and fall speed. When your opponent is put in a tech situation, they can do multiple options, but they all end up to be in 3 positions. Lab out what is best in different situations. Honestly, DB is kinda the best because it is the only move that Marth has that can interrupt his dash, and it has immense kill power, sometimes even as much as tipper Fsmash depending on rage.
It might help to know how I play, or rather, how I am trying to play(I am a pretty aggro Marth, and I am working on patience). So the TL;DR of how I play Marth is a rather simple concept, but with almost 80 match-ups now, and many different play styles within those match-ups, it can be complex. This is more a summary of what you want to do with everyone, again, for each matchup you need to be more complex.
Lets start with neutral, because that is where everything revolves around this character. When playing Neutral you want to be just outside of your opponent's range, or your range depending on the range of each respective character. If you have more range, then outside of your range, if they have more range, then outside their range. Most of the time this is roll distance away. What you are looking for is a mistake so that you can punish. A mistake can be a whiff, mispacing, airdodge, or not attacking. Congratulations you have won neutral. Now your goal is to press your advantage hard, but do not overextend. Now you want to do combos, strings, juggles, tech chases, ledgetraps, edgegaurds, or kill them, depending on the situation. Remember, go for sours in neutral to confirm into tippers, but if you get a tipper, good job, capitalize off of it. With disadvantage, save your jump when necessary, don't press buttons too much, and try to get back to ledge, or reset neutral.
TL;DR of the whole thing: Learn MUs, incorporate patience, learn Tech Chasing, get good at tippering Dancing Blade, and read the last paragraph, I can't tl;dr that because it is too important. I will also give you a video that I think will help. I hope you found this helpful.
https://www.youtube.com/watch?v=KhdBFFQPK8w